This invention relates generally to the treatment of certain diseases by tissue ablation wherein electromagnetic energy from an antenna in an ablation catheter heats tissue sufficiently to cause necrosis and a separate temperature sensing antenna in a temperature probe placed in a body passage or cavity adjacent to the ablation site measures tissue temperature to enable the operating surgeon to avoid overheating tissue during the ablation procedure. It relates especially to method and apparatus enabling the surgeon to align the two antennas to optimize that temperature measurement.
In a typical cardiac ablation procedure, an antenna catheter is used to resistively heat heart tissue, usually at the left side of the heart, sufficiently to intentionally damage the target tissue in order to cure a potentially fatal heart arrhythmia. Typically, heating the tissue to a temperature in excess of 70° C. for 30-60 seconds is sufficient to cause tissue necrosis. During treatment, electromagnetic energy, usually in the RF frequency range, is applied between the tip of the antenna catheter and a ground plate removably affixed to the patient's back, creating an electrical circuit. The point of highest resistance in this circuit, normally the interface between the catheter tip and the heart tissue, is the region which heats the most and thus may cause intentional, irreversible damage to the heart tissue to correct the arrhythmia.
Anatomically, the esophagus is very close to, and may even contact, part of the left atrium. Indeed, the average distance between the endocardial surface of the left atrium and the anterior surface of the esophagus is only in the order of 4.4+/−1.2 mm. Thus, ablating certain regions of the left atrium to treat various arrhythmias in the heart can unintentionally cause thermal damage to the esophagus, often with severe consequences.
In order to prevent such overheating, a temperature probe may be positioned in the patient's esophagus adjacent to the ablation site in the heart. One conventional temperature probe carries conventional point source temperature sensors such as thermocouples, thermistors or the like to monitor, and ultimately prevent the overheating of, the esophagus wall by cutting off or reducing the power delivered to the ablation catheter; see, e.g., US2007/0066968.
Another type of temperature probe developed only recently is disclosed in Provisional Application No. 61/145,800, of even date herewith, the entire contents of which are hereby incorporated herein by reference. That probe incorporates a microwave antenna which is connected to an external receiver in the form of a radiometer. The radiometer detects the thermal emissions picked up by the antenna in the probe which reflect the temperature of the tissue being examined and produces corresponding temperature signals to control a display which displays that temperature. During ablation, that apparatus can measure the temperature at depth in the esophageal tissue which is in close proximity to the ablation site in the patient's heart. That measurement can then be used to prevent unintentional thermal damage to the esophagus or other body passage.
As described in the above provisional application, a temperature probe using microwave radiometry provides definite advantages in that it can measure temperature at depth in the passage wall to avoid thermal damage thereto enabling the operating surgeon to adjust the power to the ablation catheter as needed to provide sufficient heating of the heart tissue to cause necrosis, but not enough to result in surface charring of that tissue that could cause a stroke and/or the formation of microbubbles (popping) that could rupture the heart vessel wall. Also, such radiometric sensing allows accurate measurement of tissue temperature even when cooling is being provided.
However, in order to optimize the accuracy of the temperature measurement provided by the temperature probe, it is desirable that the antenna therein be aligned properly with the antenna in the ablation catheter. Until now, there has been no means in the prior apparatus of this type to enable the operating surgeon to verify that the two antennas are indeed in alignment. Resultantly, in some instances, the temperature measurements may not be accurate enough to avoid thermal damage to tissue and in others, the ablation procedure may take too long because of tissue underheating.